In the field of work machines such as hydraulic excavators, recent years have seen the development of hybrid work machines that use both an engine (diesel engine) and an electric motor in order to, for example, improve fuel consumption, improve exhaust characteristics, or reduce noise, and some of them have been put to practical use. Patent Document 1 discloses such a hybrid construction machine.
In the hybrid construction machine disclosed in Patent Document 1, a generator-motor is provided as an auxiliary power source for a hydraulic pump driven by an engine. If the demanded torque of the hydraulic pump is larger than the engine output torque, the electric power of a battery is used to operate the generator-motor as a motor, thereby compensating for the shortage of the engine output torque. If the charge amount of the battery becomes insufficient, torque reducing control is performed on the hydraulic pump to cause the engine to coercively generate surplus torque for the engine and to operate the generator-motor as a generator, thereby rapidly charging the battery.